The present invention is directed towards semiconductor packaging techniques, and more specifically, towards the fabrication of wafer-level packages.
Many semiconductor devices are sensitive to contamination, humidity, and other such environmental factors. To protect them from harm, the devices must be put in a hermetically sealed package. In the past, a device had to first be cut or otherwise separated from its wafer before it could be put into a package. In wafer-level packaging, packaging is performed while the device remains on the wafer. In this fashion, hundreds or thousands of packages can be created simultaneously, and then separated by sawing or other means.
FIG. 1 illustrates an exemplary wafer-level package 101. A first wafer 102 has a gasket 103 bonded to a second wafer 107, creating a hermetically sealed environment 109 between the two wafers to protect a device 111. Only a portion of wafer 102 and wafer 107 are shown in the figure, but there can be hundreds or thousands of such wafer-level packages formed simultaneously between the two wafers. The gasket 103 is created by depositing material such as metal or a polyimide onto wafer 102 in the desired gasket shape. The two wafers are then bonded together at the gasket. This method is effective in creating hermetically sealed environment 109. However, the deposited material is not very rigid, and is prone to deformation when pressure is applied during the bonding process. Since much of the applied pressure is used up by gasket deformation, very little of the applied pressure actually transfers to the bond juncture itself. Therefore, more pressure has to be applied for a longer period of time to create the bond. Furthermore, the process of depositing material creates an irregular gasket bonding surface, which causes problems during bonding. Finally, it is difficult to precisely control the geometry of the gasket, since the deposited material tends to shrink or change shape during the deposition process.
In accordance with an illustrated preferred embodiment of the present invention, a gasket for a wafer-level package is carved out from the wafer material itself. The wafer is typically made of silicon, although materials such as glass, 3-5 compounds, ceramics, plastics or other materials can be used. Silicon is extremely rigid, and therefore an ideal gasket material. When pressure is applied during bonding, the silicon gasket will not deform, and the applied pressure is concentrated at the bond juncture. As a result, it takes less force and/or time to bond the wafer-level package. Furthermore, the original wafer surface is manufactured to be extremely flat. Since the gasket surface is the original wafer surface, the gasket will also have an extremely flat surface, which facilitates a uniformly tight bond.
The present invention also uses existing silicon etching technology that offers high precision in both etch depth and gasket width when carving out the desired shape. This allows greater control over the dimensions of the hermetically sealed cavity. Also, due to the high strength of silicon, the gaskets can be made narrower than in the pastxe2x80x94usually no more than 10 xcexcm wide. The surface area of the gasket determines the amount of pressure on the bond juncture during bonding for a given bonding force. Since a narrower gasket has less surface area, it is able to concentrate more of the applied bonding force at the bond juncture to create a strong seal.
Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.